To allow straight-line stability and relaxed cruising their mountings must be tilted at a particular angle (castor). These three angles constitute the vehicle geometry.

What can go wrong?
Even in normal use, the alignment (of the front wheels in particular) can be put out of parallel by impact (striking kerbs, bumps, holes and stones) or by wear on tyres or parts of the steering system, such as ball-joints. Even if the misalignment is so slight that it cannot be detected by the naked eye, it causes many negative effects. Whether the wheels are pointing inwards or outwards, even by as little as a quarter of a degree, they are heading in different directions, causing the tread to scrub and wear both very rapidly and unevenly, increasing rolling resistance to the detriment of both performance and fuel consumption, and as each wheel fights for supremacy the steering will tend to wander or pull to one side. This is sufficiently common that all vehicles have adjustable rods in their steering system, to allow alignment to be corrected quickly and easily. While changing the alignment is simple, adjusting it to exactly the right position requires sophisticated equipment and skilled technicians.

What about camber and castor?
These settings are less fragile, but impacts severe enough to “bend” steering, suspension or axle components, or extreme wear or partial failure of these components, can put camber and castor geometry out of true.

Incorrect camber angles cause severe steering pull and uneven tyre wear; they can affect both straight-line and cornering stability. The effect of castor angles is to ensure the vehicle naturally goes and stays straight, especially at high speeds, unless it is being deliberately steered. If these angles are incorrect, natural stability is
lost and the vehicle tends to wander off-line, making driving more difficult and stressful. Some vehicles have a ready “minor” adjustment for camber and castor; others can be rectified by addition of shims. Some vehicles cannot be adjusted and require an engineering workshop and possible replacement of major components to remedy the problem. In all these cases, the first step is to be able to diagnose and precisely measure the degree of error.

What is wheel balance?
At a cruising speed of 100 kph, your car wheels rotate 15 times per second! At this spin-speed, even the slightest anomaly in shape or weight (balance) will cause them to vibrate. The imbalance may be so slight that it is undetectable to the driver, especially on a poor road surface: or it can be so severe that, within certain speed bands, it causes the steering wheel to shake quite violently.

Why is balance important?
Whether the imbalance is slight or severe, vibration causes discomfort to the driver, increases tyre wear, reduces handling integrity, adds rolling resistance and thus increases fuel consumption, and over time can damage other mechanical components and the bodywork. Vibration means that as the tyre wheel assembly is going round and round, it is also shaking very slightly from side to side - 500 times every kilometer, many millions of times in the life of the tyre. This not only scrubs the tread causing much faster wear, but also transmits millions of tiny hammer blows to the wheel bearings, ball joints, steering mechanism, bushes, mountings...and shakes every weld and nut and bolt and seal in the vehicle, causing rattles and ageing.

How is balance corrected?
Special equipment is used to establish the exact location and degree of weight variations in the wheel-tyre assembly, and small off-set weights are attached to the wheel rim to correct the errors. The process takes just a few minutes per wheel.
The exact size and position of the weights is crucial. For perfect balance, accuracy to within one gram and one milimetre is necessary. Conventional wheel-balancing equipment is accurate enough to remedy severe vibration, but only more sophisticated computerized systems - operated by skilled technician - can get it absolutely right and prevent not only the vibrations you can feel, but also those you can’t.